1. Field of the Invention
The present invention relates to transient signal detectors, and in particular, transient signal detectors for use with subscriber line interface circuits for detecting and compensating transient signals while allowing such transient signals to settle.
2. Description of the Related Art
Many types of circuits require some form of transient signal detection whereby signals within the monitored signal path which exceed some predetermined voltage threshold, and which are frequently transient in nature, can be reliably detected. Rapid and reliable detection of such signals allows such circuits to then either filter out or attempt to compensate or suppress such transient signals.
One such type of circuit is that of a subscriber line interface circuit (SLIC) for telecommunications. Signal transients in a telecommunications environment are quite common. For example, aside from any transient signals introduced by the operating environment (e.g., electrical motors, lightning strikes, ground loops, etc.), transient signals due to transitions between on-hook and off-hook conditions and normal dialing impulses caused by older rotary telephones are often encountered. In many applications such transient signals can cause the circuit operation to become, at best, unreliable and, at worst, fail.
Conventional transient signal detectors generally rely upon the detection of the level, or magnitude, of the transient signal. Typically, when the transient signal magnitude exceeds some predefined threshold, the transient detector indicates the presence of the transient signal, and continues to do so for as long as the transient level remains higher than the threshold. Once the transient signal decreases back below such threshold, the transient detector changes state to indicate that the transient signal has terminated. However, since the threshold value at which the transient detector switches between its positive and negative transient indication states has a non-zero value, even when the transient signal has fallen below such threshold and the transient detector indicates that the transient signal has terminated, the transient signal nonetheless remains, albeit at a reduced level, for some period of time until it actually settles out, or decays, completely. In other words, a positive indication of the presence of a transient signal in a conventional transient signal detector lasts only as long as the transient signal exceeds the predefined threshold, and once the transient signal ceases to exceed such threshold, albeit with some amount of magnitude-dependent hysteresis, it is then identified as having settled when, in fact, some residual amount of transient signal energy remains for some period of time thereafter.
Accordingly, it would be desirable to have a transient signal detector which, notwithstanding a significant reduction in magnitude of the transient signal, maintains a positive indication of the presence of a transient signal for a period of time sufficient to allow the transient signal to more completely settle out.